The oxidation of cumene to cumene hydroperoxide (CHP) and its subsequent acid catalyzed decomposition results in three products that have value either as products in and of themselves or for recycle. These are phenol, acetone and alphamethylstyrene (AMS). In addition, cumene is generally present in the CHP fed to the process, either as residual from the cumene oxidation step or added as a diluent. This cumene is recovered for recycle to the oxidation step.
In addition to these several products that have value, the oxidation of cumene to CHP and its subsequent decomposition results in a multitude of impurities that must be removed from the product phenol and acetone, as well as the AMS and cumene. Two impurities of particular importance in phenol production are acetol (hydroxyacetone) and methylbenzofurans. Both of these impurities lead to unacceptable color on further processing of phenol, such as in the production of bisphenol A.
Previous patents, such as U.S. Pat. No. 5,414,154 and U.S. Pat. No. 6,388,144, both of which are incorporated herein by reference in their entirety, have disclosed methods for removal of methylbenzofurans from phenol by thermal treatment in the presence of an acidic resin or solid superacid catalyst by converting methylbenzofurans to products boiling higher than phenol and subsequently distilling the phenol away from these products. Both of the above patents further disclose the necessity of removing acetol from the phenol stream prior to the thermal treatment to avoid the production of additional methylbenzofurans by condensation of the acetol with phenol. A typical method for removing acetol disclosed in the above patents involves treatment of the phenol stream with an amine. Such treatments, while effective, are costly in that they require an additional raw material, amine. Further, the presence of the amines in the effluent streams sent for burning results in an increase in NOx emissions.
Other processes have dealt with the problem of acetol by means of a distillation step designed to separate phenol and acetol. Such a process is described in U.S. Pat. No. 4,251,325. In the process described therein, phenol and heavier boiling products are separated from an overhead stream comprising acetone, water and products lighter than acetone. The resulting phenol stream, containing 5 to 15 percent by weight of a combination of cumene and AMS is fed to a mid-point of a distillation column. Conditions in the column are maintained such that the bottoms product of the distillation is a phenol stream containing less than 30 ppm of acetol. While effective, this method is energy intensive.
U.S. Pat. No. 4,340,447 describes a process wherein acetol is carried forward to an acetone purification column in concentrations of up to 4000 ppm. Vaporized acetone containing acetol is fed to the acetone purification column. A caustic material is fed to the column, at a point above where the acetone vapor is fed, to condense acetol and aldehydes to heavier boiling compounds. Purified acetone is removed as a side stream at a point between the acetone vapor feed and the caustic feed point. An overhead stream of less pure acetone is recycled to the column. Again, while this method is effective in producing pure acetone, the presence of significant amounts of acetol results in the fouling of the distillation column with polymerization products.
Therefore, there remains a need for an energy efficient method to remove acetol and methylbenzofurans from phenol. Such a method would not make use of expensive reagents such as amines to remove acetol, and would avoid the production of significant amounts of additional methylbenzofurans by condensation of acetol with phenol. Such a method would also limit distillation column fouling.